I think that you are missing the semicolon at the end of your query
$result=mysql_query("UPDATE $usertablea SET make='$make', model='$model', engine='$engine', yearletter='$yearletter', year='$year', colour='$colour', mot='$mot', tax='$tax', mileage='$mileage', ad_text='$ad_text', price='$price' WHERE ID='$ID'");